1. Field of the Invention
The present invention relates to inductive sensing systems, including systems based on resonant inductive sensing, such as for detecting position or proximity, or state or condition, of a target.
2. Description of the Related Art
An eddy current sensor is a position detecting system that commonly includes an inductor, a target, and a processing circuit. The inductor, which is typically implemented with a coil, senses changes in a time varying magnetic field that result from the movement of the target within the time varying magnetic field, while the processing circuit responds to changes in the time varying magnetic field that are sensed by the inductor.
For example, a proximity switch is a type of eddy current sensor that opens or closes a switch when an approaching target passes a point that lies a predetermined distance from the coil. Further, the proximity switch closes or opens the switch when a receding target passes the point that lies a predetermined distance from the coil.
The target has a surface that both faces the coil and lies in a plane substantially perpendicular to the longitudinal axis of the coil. The surface of the target has a center point that is aligned with the coil so that the longitudinal axis of the coil passes through the center point. A longitudinal distance between the target and the coil is measured along the longitudinal axis of the coil, between the coil and the center point.
In operation, the longitudinal distance between the coil and the target surface decreases or increases as the target moves. When the target is exposed to the time varying magnetic field, the time varying magnetic field induces eddy currents in the surface of the target, which causes the time varying magnetic field to lose power.
The power loss due to the eddy currents scales linearly with the total amount of magnetic flux that the target receives from the coil. Also, the mutual inductance between the target and the coil scales linearly with the same total amount of flux received by the target. Therefore, a proximity switching threshold that is based on power loss, inductance, or both corresponds to the amount of magnetic flux that is received by the target.
When the longitudinal distance between the target and sensor decreases, the magnetic flux received by the target increases which, in turn, increases the magnitudes of the eddy currents. The increase in the magnitudes of the eddy currents causes the time varying magnetic field to lose more power. A proximity switch detects the loss of power of the time varying magnetic field, and causes the switch to change states when the power loss of the time varying magnetic field falls below the threshold power level.
Since the threshold power level corresponds with the predetermined longitudinal distance, the proximity switch causes the switch to change states when the longitudinal distance between the target and sensor passes the point that lies the predetermined longitudinal distance from the sensor.
Similarly, when the longitudinal distance between the target and sensor increases, the magnitudes of the eddy currents decrease. The decrease in the magnitudes of the eddy currents causes the time varying magnetic field to lose less power. The proximity switch detects the loss of less power, and causes the switch to again change states when the power of the time varying magnetic field rises above the threshold power level.
Since the threshold power level corresponds with the predetermined longitudinal distance, the proximity switch causes the switch to again change states when the longitudinal distance between the sensor and a receding target passes the point that lies the predetermined longitudinal distance from the sensor.
The proximity switch, including the coil, the target, and the processing circuit, are connected to a support structure so that the target and the sensor can move with respect to each other. The processing circuit generates a signal in response to the measured energy loss or mutual inductance. In addition, the processing circuit generates a switch signal that controls the open or closed state of the switch in response to the energy of the time varying magnetic field rising above and falling below the threshold power level.
One of the disadvantages of the proximity switch is that the accuracy of this sensing technique decreases rapidly with the longitudinal distance to the coil, with the usable sensing range being limited to approximately 50% of the coil diameter. The power loss that is sensed by the coil corresponds to the total magnetic flux that is received by the target as the target moves along the longitudinal axis of the coil.
However, the magnetic flux density changes in a highly non-linear fashion along the longitudinal axis. Since the magnetic flux density changes in a highly non-linear fashion, the power loss that is sensed also has a highly non-linear dependence on position. As a result, the sensing accuracy decreases rapidly with longitudinal distance to the coil. Thus, there is a need for a position sensing technique that has position independent accuracy and an arbitrary sensing range.